Attempts to improve the performance of polymer compounds have hitherto been made in various ways. For example, to increase the refractive index of polymer compounds, introduction of an aromatic ring, halogen atom, or sulfur atom has been attempted. In particular, episulfide polymer compounds and thiourethane polymer compounds having introduced sulfur atom are used in high-refractive-index lenses for eyeglasses.
However, it is difficult to design a material with a refractive index higher than 1.7 by using a polymer alone. Thus, a method using fine inorganic particles has been known as the most effective method that can achieve a further increase in refractive index.
In this method, a polymer and fine inorganic particles are mixed to achieve a high refractive index. A common technique for the mixing is to mix a solution of the polymer and a dispersion of the fine inorganic particles. In this case, the polymer serves as a binder to stabilize the dispersion state of the fine inorganic particles without impairment of the dispersion state.
It has been reported that a hydrolysis-condensation product of a hydrolyzable silane or a polyimide can be used as the binder polymer as described above. For example, a technique has been reported in which a hybrid material prepared by mixing a hydrolysis-condensation product of an alkoxysilane and an inorganic oxide dispersion containing dispersed zirconia or titania is used to achieve an increase in refractive index (see Patent Document 1). There has also been reported a technique in which a hybrid material containing a polyimide and dispersed titania, zinc sulfide or the like is used to achieve an increase in refractive index (see Patent Document 2).
Although these hybrid materials are deliberately designed to achieve a high refractive index, any high-refractive-index composition having a refractive index higher than 1.7 and capable of providing a film thickness of more than 1 μm has not yet been obtained. Furthermore, no investigation as to the storage stability of coating compositions has been conducted.
In recent years, high-performance hybrid materials are increasingly required for development of electronic devices such as liquid crystal displays, organic electroluminescence (EL) displays, optical semiconductor (LED) devices, solid-state image sensors, organic thin-film solar cells, dye-sensitized solar cells, and organic thin-film transistors (TFT).
Specific examples of required properties include heat resistance, transparency, high refractive index, large film thickness, crack resistance, and coating composition storage stability.
Mixing of a hydrolysis-condensation product of a hydrolyzable silane, namely a polysiloxane, with fine inorganic particles generally results in a film having high heat resistance and high transparency. A common approach to increasing the refractive index of the film is to reduce the content ratio of the polysiloxane which has a low refractive index and increase the content ratio of the fine inorganic particles which have a high refractive index. A common approach to increasing the film thickness is to increase the solids concentration of the coating composition containing the polysiloxane and the fine inorganic particles.
The approach to increasing the film thickness by increasing the solids concentration of a coating composition is based on the theory that the increase in solids concentration can yield an increase in viscosity of the coating composition and that a film formed from the coating composition of high viscosity can be a thick film. This approach is suitable when an organic polymer such as an acrylic polymer is used.
However, the viscosity of a coating composition containing a polysiloxane and fine inorganic particles is not readily increased by increasing the solids concentration; namely, the viscosity of the composition remains low at the time of film formation, and thus it is difficult to achieve a large film thickness. Furthermore, when attempting to achieve a high refractive index of the composition along with a large film thickness, the content of the fine inorganic particles in the coating composition needs to be increased, as described above, to increase the refractive index, which makes it more difficult to increase the viscosity of the coating composition. This is because fine inorganic particles selected for use in a coating composition in the art are well dispersed in the coating composition and such fine inorganic particles of high dispersibility make it difficult to increase the viscosity of the composition.
It may be possible to increase the thickness of a film of a composition containing a hydrolysis-condensation product of a hydrolyzable silane and fine inorganic particles by increasing the dispersed particle diameter of the fine inorganic particles and thus increasing the viscosity of the coating composition.
However, this approach, which indeed allows an increase in film thickness, has the disadvantage of deteriorating the storage stability of the coating composition and cannot ensure a long product life of the composition. A coating composition with poor storage stability is, for example, not resistant to viscosity increase during storage of the coating composition, which leads to an undesired situation such as where the film thickness cannot be controlled to a target thickness or where the composition turns into a gel and becomes unusable.
As seen from the foregoing description, a technical trade-off exists between achievement of increase in thickness of a film of a composition containing a polysiloxane and fine inorganic particles and achievement of high storage stability of the composition.
As described above, there has been no report of any coating composition that comprises a polysiloxane and fine inorganic particles, that can be formed into a highly heat-resistant, highly transparent coating film capable of exhibiting a high refractive index and having a large thickness and crack resistance and excellent storage stability, or of any process for producing such a composition.